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Science Highlights

Slow-spreading ridges could be major oceanic iron contributor

A large dissolved iron- and manganese-rich plume has been detected by Saito and co-authors over the slow-spreading southern Mid-Atlantic Ridge. This discovery calls into question the assumption that deep-sea hydrothermal vents along slow-spreading ridges were negligible contributors to the oceanic iron inventory. This result urges reassessment and a likely increase of the contribution of hydrothermal vents to the supply of iron.

13 Saito NobleFigure: A zonal section of dissolved iron in the South Atlantic. The higher iron concentrations (in warm colours red, orange) reveal a large plume at ∼2,900 m depth and 2 km in height.
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Dissolved iron in the tropical North Atlantic Ocean

Surprisingly high dissolved iron (dFe) values have been found in the subsurface oxygen minimum waters of the tropical North Atlantic from the Cape Verde Islands west to 42°W, reaching concentrations of 1.0-1.5 nmol/kg. Fitzsimmons' and co-authors results suggest that biological regeneration is a more consistent explanation for this enrichment than sedimentary diagenetic dFe mixed out from the African margin. The authors also mention the limits of artificial fertilizations, since their data demonstrates that high levels of surface Fe enrichment might be relatively ineffective as a means of removing carbon from the surface ocean.

13 Fitzsimmons lFigure: This figure shows vertical dFe distribution in the upper 1000 m of the tropical North Atlantic Ocean. High dFe concentrations are in warm colours (red, orange,etc). Black dots represent sample locations.
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GEOTRACES: Get the right data!

Measuring concentrations of many constituents at the nano- to picomolar levels in seawater is a difficult challenge, requiring thorough intercalibration between the different laboratories participating to the same programme. GEOTRACES is but one example of a programme that has intercalibration as an essential component of its field-based efforts. The standing GEOTRACES Standards and Intercalibration Committee (S&I) chaired by Greg Cutter was initiated to ensure precision and accuracy throughout the programme's life. To facilitate consistent sampling and sample handling procedures between cruises and participating laboratories, the S&I Committee in collaboration with the elemental coordinators created a set of written protocols for each suite of trace elements and isotopes. Cutter's paper emphases on the importance of rigourous intercalibration protocols to get the right data.

13 Cutter
Figure: Participants on the first leg of the 2008 intercalibration cruise in the North Atlantic Ocean abord R/V Knorr

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First combined distributions of dissolved Neodymium (Nd) - Hafnium (Hf) concentrations and isotope compositions from the Baltic Sea

The central Baltic Sea is a marginal brackish basin, in which anoxic bottom waters prevail either seasonally or permanently and which is surrounded by continental landmasses comprising a large variety of compositions and ages of rocks. This allows the investigation of water mass mixing using radiogenic isotope compositions of Nd and Hf, as well as the geochemical cycling of these elements in the water column. Tianyu Chen and co-workers present the first combined distributions of dissolved Neodymium (Nd) and Hafnium (Hf) concentrations and their isotopic compositions from the Baltic Sea obtained during a cruise onboard the Polish RV Oceania in the frame of a process study of the GEOTRACES program.

2013 Chen FrankFigure: Neodymium-hafnium isotope systematics of the central Baltic Sea, in comparison with published Nd-Hf isotope data of terrestrial rocks, seawater (<0.45 μm), and Fe-Mn crusts (for sources of the data see publication).

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Are the mid oceanic ridges a source of radiogenic neodymium input?

An unexpected radiogenic value of neodymium (Nd) isotopic composition observed by Catherine Jeandel and colleagues in the Deep Pacific Water raises the question of the role of mid ocean ridges in modifying the oceanic Nd signature.
GEOTRACES researchers measured three vertical profiles of rare earth element (REE) concentrations and Nd isotopic compositions in the remote southeast Pacific Ocean. Close to the coast, Nd isotopic compositions were clearly more radiogenic than in the open ocean, suggesting that boundary exchange process is releasing lithogenic rare earth element from the volcanic Andes. A surprisingly radiogenic value was also observed at 2000m in the Upper Circumpolar Deep Water (UCDW) that commonly have low radiogenic values (εNd values around -6, see figure below). Exchange processes related to hydrothermal activity are suspected to produce this increase in the value of neodymium isotopic compositions in the vicinity of the East Pacific Rise. These results provide some guidance for higher resolution studies planned in this region by the international GEOTRACES program.
 

13 Jeandel ridges
Figure: Vertical profiles of εNd values at the 3 stations. An extended scale for the upper layers of station UPX is proposed. The doted line marks the εNd value of -6. Acronyms recall the water mass names. The most striking features are 1) the radiogenic values along the Chilian coast, reflecting local inputs and 2) the relatively high value for UPCW at station GYR, which is suspected to reflect an hydrothermal influence. 
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to see the figure in high resolution. Source author's manuscript.

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